Winter project, who knows about epoxy grout?

I was going to use black thin-set for grout, but I've heard about epoxy grout
though not enough to know if I can use it. I tested thin-set on the bottom.
It's ok, but this will be outdoors half the year, so I might want something even stronger.

Also, because of all the ups and downs, it's a pretty crude surface, a squeegee
immediately got roughed up by all the sharp edges and corners.
When I do the whole thing, I was going to try to get it on very thickly
and wait an hour to sponge it, and use a dremel to "dig out" the mirrors>
Any advice here from any of you adepts would be hugely welcome!